docs: def _getNextId(self):
          """
          Returns an unique id.
  
          By default, generates pool of UUID in increments
          of ``UUID_POOL_GEN_SIZE``. Could be overridden to
          provide custom ID generation.
  
          :return: generated unique "on the wire" message ID
          :rtype: str
          """
          if not self._uuids:
              for _ in range(self.UUID_POOL_GEN_SIZE):
                  self._uuids.append(uuid.uuid4().bytes)
          return self._uuids.pop()